I found this app, and love the ability to add a song, and play along/record to it - mixdown and share. Exactly what I was looking for.
However - I'm using an iPhone 6S, with an iRig HD (24/96 supported by firmware/hardware). I use a couple options, although I don't care for many of the bass amps options in the software - AmpliTube and JamUp. I've tried garageband as well - but I want to record bass, not auto-bass or some midi option - it seems to be more than I really want or need at this time.

I did find out how to add a track, and use Amplitube and JamUp - in stereo no less - and at either 16 or 26 bit; I can use 44kHz with no issues. When I up the sample rate - actually anything other than 44 - I get distortion and basically an unusable signal.

When using an IAA plugin being hosted by multitrack, the plugin must be created at the hardware samplerate. Usually this shouldn't be an issue, multitrack will always create the plugin at the hardware samplerate and then place samplerate converters around it, if the project samplerate doesn't match the hardware samplerate. Additionally it will remove them or add them back in whenever the hardware rate changes. But it seems that some plugins don't work too well with the converters and pulling them with odd buffer sizes. Further complicating the problem is the iPhone having a 48 kHz hardware samplerate when using the internal hardware and 44.1 kHz with headphones.

My advice is to use the standard samplerates (44.1 kHz in most cases, 48 kHz for bare iPhones) as that is much easier on the CPU and the sound quality will improve because to less conversions. There is usually no reason to go higher than this, honestly the higher samplerates are all marketing hype unless you're recording frequencies higher than 20 kHz and everything else in the chain supports this as well.